Speaking from Capitol Hill, Sanders say he had just learned that the shooter, identified as James T. Hodgkinson, was a volunteer during Sanders' 2016 presidential campaign.

"I am sickened by this despicable act," Sanders said. "My hopes and prayers are that Representative Scalise, congressional staff and the Capitol Police Officers who were wounded make a quick and full recovery. I also want to thank the Capitol Police for their heroic actions to prevent further harm."

Michael Briggs was the communications director for the Sanders campaign. He told KCCI “Our computer records show that he volunteers for one debate watch party in Iowa. He was not on the payroll. We never interacted with him. I talked to people on the Iowa campaign this morning who have never heard of him.”

Nick Carter, the political outreach director for the campaign, said he also was not familiar with Hodgkinson. “Many, many, many thousands of people came through the Sanders campaign offices. This individual was not on our radar."

Hodgkinson, 66, owned a home inspection business in Belleville, Illinois. Law enforcement officials say he opened fire at a baseball field in Alexandria, Virginia, shortly after 7 a.m. Wednesday.

Officials say Hodgkinson was carrying an assault weapon similar to an M-4 and a handgun. Both are being traced by the Bureau of Alcohol, Tobacco and Firearms. The FBI has taken over the investigation.

Court records show Hodgkinson was arrested in 2006 on a misdemeanor charge of battery.